Interacting chains of orbital polarons in the colossal magnetoresistance material La1-xSrxMnO3 revealed by spin and lattice dynamics

The origin of the effect of "colossal magnetoresistance" (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudocubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x <= 0.5) and various temperatures and report a lattice dynamics study at x(0) = 0.2, representative of optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum E-n(mag)(q) with n levels, characteristic of the internal excitations of "orbital polarons" defined by Mn3+ neighbors surrounding a central Mn4+ hole. Because of its hopping, the hole mixes dynamically all the possible orbital configurations of its surrounding Mn3+ with degenerate energies. The E-n(mag) values indicate a lifting of orbital degeneracy by phonon excitations. The n value and the q range are used to characterize these orbital polarons in direct space. At x = 0.125 and x = 0.3 the spectrum reveals two-dimensional polarons coupled by exchange and three-dimensional "free" polarons, respectively, with sizes l = 1.67a <= 2a in all bond directions. At x(0) = 0.2, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size l = 2a with a periodic distribution over approximate to 3a and an interaction energy approximate to 3 meV. At T <= T-c the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a nonmetallic matrix may be crucial for CMR.